1. Field of the Invention
The present invention relates to an optical transmitter apparatus which controls an output level when a digital electrical signal input, continuously or in bursts, is converted into a light signal.
2. Discussion of the Related Art
FIG. 13 discloses an optical power outputting apparatus as an example of a conventional light emitting control apparatus. This apparatus has been described on page 55-60, volume 11 of the technical report OCS-93-53, 1993 of the Institute of Electronics, Information and Communication Engineers of Japan. In FIG. 13, the optical power outputting apparatus is comprised of a buffer circuit 1, a voltage-to-current converter 2, a light emitting element 3, an optical fiber 4, a monitoring circuit 5, a first peak detection circuit 6, a differential amplifier 7, a second peak detection circuit 8, a control circuit 9, and a light signal off detection circuit 10.
Next, the operation of this apparatus is described.
First, an automatic power control operation (hereinafter referred to as the APC operation) is described. It is assumed herein that an input signal has low and high levels.
After having been reformed in its waveform via the buffer circuit 1, the input signal is divided into two branched signals. The first of the two branched signals is outputted to the voltage-to-current converter 2. In the voltage-to-current converter 2, a current signal which is turned on and off corresponding to the input signal's on/off state is generated. The current signal drives the light emitting element 3 to output a light signal which is sent to the optical fiber 4.
A fraction of the optical output of the light emitting element 3 is converted into a monitoring signal via the monitoring circuit 5. The monitoring signal is then input into the second peak detection circuit 8. In the second peak detection circuit 8, a peak value of the output of the monitoring circuit 5 is detected and is outputted to the differential amplifier 7.
The second of the branched signals from the buffer circuit 1 is input into the first peak detection circuit 6. In the first peak detection circuit 6, a peak value is detected, and outputted to the differential amplifier 7. In the differential amplifier 7, a difference of the peak values between the first peak detection circuit 6 and the second peak detection circuit 8 is detected, amplified, and input into the control circuit 9. The amplitude of the current from the voltage-to-current converter 2 is adjusted by the control circuit 9 so as to keep the difference between the inputs of the differential amplifier 7 almost zero at all times.
In the case where the input signal has only low and high levels, the output of the second peak detection circuit 8 always remains constant as long as a signal is being input into the second peak detection circuit 8. Therefore, by means of the operation described above, the optical output of the light emitting element 3 is controlled to be constant at all times.
Next, an operation for generating an alarm will be described.
The output of the second peak detection circuit 8, which executes peak detection of the optical output of the light emitting element 3 as described before, is divided and input into the differential amplifier 7 for the APC operation, and is also input into the light signal off detection circuit 10. In the light signal off detection circuit 10, a detection level from the second peak detection circuit 8 is compared with a predetermined reference level, and when the level falls to or below the predetermined reference level, a light signal off alarm is generated.
In order to avoid a failure where comparison of the data input level for the light signal off detection circuit 10 is made without data output from the second peak detection circuit 8 during the above-described operation for generating the alarm, Japanese Unexamined Patent Application HEI2-193426 discloses a related art in which, with the data input, a mark ratio of the data (used herein to mean a ratio of a mark period to a predetermined data transmission period) is detected, and an alarm reference level is set, responsive to the mark ratio. The mark level, however, does not vary with the mark ratio.
On the other hand, Japanese Unexamined Patent Application HEI4-249721 discloses another related art in which detection of data input is performed in one peak detection circuit and monitoring of the optical output is performed in another peak detection circuit. Using the outputs of these peak detection circuits, a certain result can be obtained and calculated via a predetermined mathematical algorithm. Then, by using the result, a part failure can be identified.
In the conventional optical power outputting apparatus as described hereinbefore, in order to hold the peak values constant even if intervals for signal bursts between one signal input and the subsequent signal input (hereinafter referred to as a burst interval) fluctuate, time constants of the first peak detection circuit 6 and the second peak detection circuit 8 for a peak-hold operation are set to become or exceed the maximum burst interval. Accordingly, when an initial rise of the subsequent signal input in bursts occurs after the peak detection circuits have been discharged fully with no signal input for a long time, it will take time for the first and second peak detection circuits to recognize a final peak input state. Therefore, time is necessary to stabilize the optical output.
In the conventional optical power outputting apparatus, alarm generation is controlled by a change of the output level of the second peak detection circuit 8. This creates a problem in that an alarm is generated by a no data input condition which would not normally have generated the alarm.
As already described to be another conventional related art, when the part failure is identified by comparing both input and output conditions, a peak value is usually detected after several signal input in bursts due to the poor following property of the peak detection circuit. In this case, although the properties of the peak detection circuit used for detection are not clearly defined and are exhibited differently depending upon the situation where the circuit is used, a time constant for peak detection is generally large. Therefore, the conventional optical power outputting apparatus cannot respond quickly to a change of the input state.